Determining the dead time in driving a half-bridge

ABSTRACT

A circuit arrangement for determining a temporal change of an output voltage of a half-bridge circuit during a dead time. In one embodiment, the circuit arrangement includes a first input for applying the output voltage. A capacitive network is provided having a first and a second circuit node capacitively coupled to the input, and having a terminal for a reference potential. A recharging circuit is configured, during the switched-on phase of one of a first and second switching elements, to adjust electrical potentials of the first and second nodes, the electrical potentials each being different from the reference potential. A comparator arrangement is provided that is configured during the dead time to determine a time difference between such times at which the electrical potentials at the first and second node each assume a given potential value, the time difference being a measure for the change with time of the output voltage.

TECHNICAL FIELD

The invention relates to a circuit arrangement including a half-bridgecircuit, and in one embodiment to determine the dead time betweenswitching off one of the switching elements and switching on the otherone of the switching elements of a half-bridge.

BACKGROUND

A half-bridge includes two switching elements each having a drive inputand a load path, and having their load paths connected in series witheach other between terminals for a positive and a negative supplypotential during operation. An output to which a load may be connectedto is arranged between the load paths. Using the half-bridge analternating voltage at the out-put of the half-bridge can be generatedfrom a voltage between the positive and the negative supply potential byalternatingly switching on and switching off the two switches. The twoswitching elements are driven by two drive signals each of which beingprovided to the drive input of one of the switching elements. Thesedrive signals may assume an on-level for switching on the correspondingswitching element and an off-level for switching off the correspondingswitching elements.

There are circuit applications in which the half-bridge is driven in theZVS mode (ZVS=Zero Voltage Switching). Such circuit applications are,for example, lamp ballasts, in which the load connected to thehalf-bridge includes a series resonant circuit and a fluorescent lamp(gas discharge lamp). Generally, the load in such ZVS circuits iscapable of recharging the output of the half-bridge after switching offthe first one of the switching elements and before switching on thesecond one of the switching elements, namely from the supply potential,to which the first switching element is connected to, to the supplypotential, to which the second switching element is connected to. If thesecond switching element is switched on after recharging the output,than when switching on the second switching element there is no voltagedrop across its load path. Therefore, the second switching element canbe switched not energized, or at a load path voltage of zero (zerovoltage), resulting in a reduction of switching losses. Correspondinglythe second switching element can be switched not energized afterswitching off the second switching element.

The time period between switching off one switch and switching on theother switch is referred to as dead time. This dead time should beadapted to the recharging time of the output: If the dead time is lessthan the recharging time, then zero-voltage switching is not performed,resulting in increased switching losses; if the dead time is longer thanthe recharging time, then a current direction of the load current couldrevert before the end of the dead time, resulting again in an increaseof the load path current at the individual switching element. In thiscase the switching operation would not be not energized, also.

However, the recharging time is dependent on different parameters, suchas the load or the applied supply voltage, so that a fixed adjustment ofthe dead time for driving the half-bridge switching elements is notpossible. There is therefore a need to adaptively adjust the dead timeduring operation of the half-bridge.

For these and other reasons there is a need for the present invention.

SUMMARY

A first embodiment relates to circuit arrangements for determining avariation with time of an output voltage of a half-bridge circuit duringa dead time, the half-bridge circuit including two switching elements,each of which alternatingly assuming a switched-on state and aswitched-off state, and both assuming a switched-off state during thedead time. This circuit arrangement includes: an input for providing theoutput voltage; a capacitive network having a first and a second circuitnode each of which being capacitively coupled to the input, and having aterminal for a reference potential; a recharging circuit for thecapacitive network adapted to adjust during the switched-on phase of theat least one switching element electrical potentials at the first andthe second circuit nodes that are different from the referencepotential; a comparator arrangement adapted to determine during the deadtime a time difference between those times at which the electricalpotentials at the first and the second nodes each assume a givenpotential value. This time difference is a measure for the variation ofthe output voltage with time.

A second embodiment relates to a control circuit for a half-bridgecircuit having a first and a second switching element and having anoutput. This control circuit includes: a drive signal generation circuitadapted to generate a first drive signal for the first switching elementand a second drive signal for the second switching element dependent ona dead time signal; and a dead time signal generation circuit forgenerating the dead time signal. The dead time signal generation circuitincludes: an input adapted to be coupled to the output of thehalf-bridge circuit; a capacitive network having a first and a secondcircuit node each being capacitively coupled to the input, and having aterminal for a reference potential; a recharging circuit for thecapacitive network adapted to adjust during the switched-on phase of theat least one switching element electrical potentials at the first andthe second circuit node that are different from the reference potential;a comparator arrangement adapted to determine during the dead time atime difference between those times at which the electrical potentialsat the first and the second nodes each assume a given potential value,and to generate the dead time signal dependent on this time difference.

A third embodiment relates to a method for determining a variation withtime of an output voltage of a half-bridge circuit during a dead time,the half-bridge circuit including two switching elements eachalternatingly assuming a switched-on state and a switched-off state, andboth assuming a switched-off state during the dead time. The methodincludes: providing a capacitive network having an input receiving theoutput voltage, and having a first and a second circuit node each beingcapacitively coupled to the input; recharging the first and secondcircuit nodes during the switched-on phase of the at least one switchingelement in such a manner that electrical potential settle in at thefirst and the second circuit nodes that are different from the referencepotential; during the dead time: determining a time difference betweenthose times at which the electrical potentials at the first and thesecond nodes each assume a given potential value, this time differencebeing a measure for the variation of the output voltage with time.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following examples will be explained with reference to drawings.These drawings serve to explain the basic principle, so that only thosefeatures relevant for understanding the basic principle are illustratedin the drawings. In the drawings, unless stated otherwise, samereference symbols denote same features with the same meaning

FIG. 1 illustrates by way of a block diagram the basic configuration ofa half-bridge circuit.

FIG. 2 illustrates the operating principle of a half-bridge operated inzero-voltage-switching mode (ZVS mode).

FIG. 3 by way of a block diagram illustrates a circuit arrangementhaving a capacitive network, a recharging circuit and a comparatorarrangement for determining a variation with time of an output voltageof the half-bridge.

FIG. 4 illustrates the operating principle of the circuit arrangementillustrated in FIG. 3 by way of signal timing diagrams.

FIG. 5 illustrates an implementation example of the capacitive networkand the recharging circuit of the circuit arrangement according to FIG.3 by way of electrical circuit diagrams.

FIG. 6 illustrates the operating principle of the capacitive network andthe re-charging circuit according to FIG. 5 by way of signal timingdiagrams.

FIG. 7 illustrates the operating principle of the capacitive networkaccording to FIG. 5 during a start-up or transient phase.

FIG. 8 illustrates a further example of a capacitive network and arecharging circuit of the circuit arrangement according to FIG. 3 by wayof electrical circuit diagrams.

FIG. 9 illustrates the operating principle of the capacitive network andthe re-charging circuit according to FIG. 8 by way of signal timingdiagrams.

FIG. 10 illustrates an example of a drive circuit for a half-bridge.

FIG. 11 illustrates the operating principle of the drive circuitaccording to FIG. 10 by way of signal timing diagrams.

FIG. 12 illustrates a block diagram of an example of the comparatorarrangement having two comparators and an evaluation circuit.

FIG. 13 illustrates an implementation example of the evaluation circuitaccording to FIG. 12.

FIG. 14 illustrates the operating principle of the evaluation circuitaccording to FIG. 13 by way of signal timing diagrams.

FIG. 15 illustrates, by way of a block diagram, an example of a controlcircuit for the half-bridge, the control circuit including a circuitarrangement for determining a variation of an output voltage of thehalf-bridge with time, and a transformer in the control path of one ofthe switching elements of the half-bridge.

FIG. 16 schematically illustrates an example for implementing thetransformer.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, therefore, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

It is to be understood that the features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise.

FIG. 1 schematically illustrates one embodiment of a half-bridge circuithaving a half-bridge that includes two switching elements, namely afirst switching element 101 and a second switching element 102, eachhaving a load path and a drive input. The load paths of the twoswitching elements 101, 102 are connected in series with each otherbetween terminals for a first supply potential Vin and a second supplypotential GND. The first supply potential Vin is, for example, apositive supply potential, the second supply potential GND is, forexample, a negative supply potential or a reference potential, such asground. The half-bridge further includes an output 103 that is arrangedbetween the load paths 101, 102 of the switching elements, and that, inthe example as illustrated, is formed by a node common to the load paths101, 102. In this connection it should be mentioned that between theload paths of the two switching elements 101, 102 additional elements,such as a measurement resistor (not illustrated), could be arranged.

The first switching element 101, that is connected between the terminalfor the first supply potential Vin and the output 103, will be referredto as upper switching element or high-side switch of the half-bridge inthe following, and the second switching element 102, that is connectedbetween the output 103 and the second reference potential GND, will bereferred to as lower switching element or low-side switching element ofthe half-bridge in the following. The switching elements 101, 102 aredriven to be conducting or blocking, or switched-on or off,respectively, dependent on drive signals S1, S2 that are provided totheir drive inputs. In FIG. 1 reference symbol S1 denotes a first drivesignal that is provided to the drive input of the first switchingelement 101, and reference symbol S2 denotes a second drive signal thatis provided to the drive input of the second switching element 102.

Switching elements 101, 102 can be any switching elements that areadapted to be switched-on and off by electrical drive signals, such asdrive signals S1, S2. Such switching elements are in one embodimentsemiconductor switching elements. Examples for suitable semiconductorswitching elements 101 102 for use in the half-bridge are MOSFET, in oneembodiment power MOSFET, or IGBT, in one embodiment power IGBT. MOSFETor IGBT each include a drain-source-path (which is also referred to ascollector-emitter path in an IGBT) that form their load paths, and eachinclude a gate terminal that forms a drive input. Controlling MOSFET orIGBT is performed by applying suitable gate-source voltages. Thus, drivesignals for driving MOSFET or IGBT are their gate-source voltages.

For purpose of the following explanation it may be assumed that aswitching element 101, 102 of the half-bridge is conducting, if itscontrol signal S1, S2 assumes an on-level, and that the switchingelement 101, 102 is blocking, if its control signal S1, S2 assumes anoff-level. The on-level for switching on the switching element and theoff-level for switching off the switching element, respectively, isdependent on the type of the switching element that is used. Inre-channel MOSFET or n-channel IGBT the on-level is a positivegate-source voltage, while in p-channel MOSFET or in p-channel IGBT theon-level is a negative gate-source voltage. Only for illustrationpurposes it is assumed in connection with the following explanation thaton-levels of the control signals S1, S2 are upper signal levels or“high” levels, respectively, of the control signals and that off-levelsof the control signals are lower signal levels or “low” levels of thecontrol signals.

In FIG. 1 reference symbol 60 denotes a drive circuit that generatesdrive signals S1, S2 (illustrated in dashed lines). Reference symbol Zdenotes a load (also illustrated in dashed lines) that is connected tothe output 103 of the half-bridge, and that is driven by an outputvoltage V_(HB) of the half-bridge. Via the switching elements 101, 102of the half-bridge the output 103 can either be connected to the firstsupply potential Vin, or to the second supply potential GND. Ifon-resistances of the switching elements 101, 102 are neglected, thenthe maximum amplitude of the output voltage V_(HB) equals the supplyvoltage present between the terminals for the first and the secondsupply potential. With the first switching element 101 switched on andthe second switching element 102 switch off the output voltage V_(HB) isapproximately equivalent to the supply voltage; with the secondswitching element 102 switched on and the first switching element 101switched off the output voltage is approximately zero.

A half-bridge, such as a half-bridge illustrated in FIG. 1, is, forexample, used for driving loads that as an input voltage or supplyvoltage, respectively, require a voltage that oscillates between theupper and the lower supply potential with time. Such loads are, forexample, inductive loads, such as electric motors or magnetic valves, orlamp circuits for fluorescent lamps. Such lamp circuits may include aseries resonance circuit having a resonance capacitor and a resonanceinductance, and a fluorescent lamp connected in parallel with theresonance capacitor, and therefore being in series to the resonanceinductance. For providing such oscillating output voltage V_(HB) each ofthe switching elements 101, 102 is alternatingly switched on and off ina commonly known manner, i.e., assumes alternatingly a switched-on orswitched-off state over time. The switched-on and switched-off phases ofthe two switching elements 101, 102 are phase-shifted in such a way thatat one time only one of the switching elements 101, 102 is switched-onand that between switching off one of the switching elements andswitching on the other one of the switching elements there is a deadtime for which both switching elements 101, 102 are switched-off.

This mode of controlling the two switching elements 101, 102 isillustrated in FIG. 2 by way of timing diagrams of the first and secondcontrol signals S1, S2. In FIG. 2 t11 denotes a time at which the firstdrive signal S1 assumes an on-level, and t12 denotes a time at whichthis first drive signal S1 assumes an off-level. During this time periodt11-t12 the second drive signal S2 has an off-level and keeps this levelalso for a dead time T_(D) after switching off the first switchingelement 101 until the second switching element 102 is switched on attime t13. At a time t14 the second switching element 102 is switchedoff, and the first switching element 101 is switched on again not untilafter expiration of a dead time T_(D) after switching off the secondswitching element 102, which is at a later time t15.

In FIG. 2 a timing diagram of the output voltage V_(HB) dependent on thedrive signals S1, S2 for a zero-voltage-switching mode (ZVS mode) isalso illustrated. In this mode of operation a voltage change of theoutput voltage V_(HB) during the dead time T_(D) is initiated by theload Z in such a manner that the voltage across the switching elementthat is to be switched on next becomes zero during the dead time. Thismeans, referring to the example illustrated in FIG. 2, that during thedead time between the times t12 and t13—i.e., after switching off thefirst switching element 102—the output voltage V_(HB) decreases from theupper voltage level or the value of the supply voltage, respectively, tothe lower voltage level or zero, respectively. During the dead timebetween the times t14 and t15—i.e., after switching off the secondswitching element 102—the voltage increases from the lower voltage levelto the upper voltage level. An example of a load Z that allows such ZVSoperation is a lamp circuit (not illustrated) having a series circuitwith a resonance inductance and a fluorescent lamp (gas discharge lamp)with the series circuit being connected between the output 103 and lowersupply potential GND. A further example of a load that allows ZVSoperation is a LLC converter. Such LLC converter includes a seriescircuit with a series resonance circuit having a resonance inductanceand a resonance capacitor, and a primary winding of a transformer. Inthis connection the transformer includes a secondary winding beinginductively coupled to the primary winding, a rectifier arrangementbeing connected downstream the secondary winding. The resonanceinductance may be part of a transformer, if the inductive couplingbetween the primary winding and the secondary winding is less than 1.

A zero-voltage operation of the half-bridge is basically possible if theload illustrates an inductive behavior at the drive frequency with whichthe switching elements 101, 102 are switched on and off. In a lampcircuit this is the case, if the drive frequency is higher than aresonance frequency of the series resonance circuit, or if there is astrong attenuation of the oscillator circuit through the lamp. Thelatter is the case after ignition of the lamp.

For the sake of completeness it should be mentioned that the switchingelements 101, 102 may include switching delays. These switching delayshave the effect that the switching elements 101, 102 switch ontime-delayed after an on-level of the corresponding drive signal S1, S2,and switch off time-delayed after an off-level of the correspondingdrive signal S1, S2. However, these switching delays are not illustratedin FIG. 2.

The time period within which the output voltage V_(HB) changes itsamplitude during the dead time T_(D), or within which the potential atoutput 103 is recharged will be referred to as recharging time in thefollowing. With ideal switching elements 101, 102, ideal connectinglines and an ideal load, that does not include parasitic capacitances,this recharging time would be zero. Since those parasitic capacitancesare inevitably present, this recharging time is different from zero. Incertain applications, such as use of the half-bridge in a lamp ballast,it may be desired to extend the recharging time beyond the period asgiven by the parasitic capacitances. For this purpose a furthercapacitance is connected in parallel with one of the half-bridgeswitching elements 101, 102. In the circuit according to FIG. 1 such anoptional capacitance 104 (illustrated in dashed lines) is exemplaryconnected in parallel with the lower switching element 102.

For operating the half-bridge it is desired to adapt the dead time T_(D)to the recharging time in such a way that the switching element that isto be switched on next is switched on after the end of the rechargingprocess, with a delay that is as small as possible. In an ideal case thedead time corresponds to the recharging process or is somewhat longerthan the recharging time, such as 2% to 20% longer than the rechargingtime.

A circuit arrangement and a method for adaptively adjusting the deadtime of a half-bridge operating in ZVS mode will be illustrated in thefollowing. FIG. 3 illustrates a block diagram of such circuitarrangement 10. This circuit arrangement 10 includes an input 11 thatmay be coupled to the output 103 of the half-bridge (illustrated indashed lines) and through which the output voltage V_(HB) of thehalf-bridge can be provided to the circuit arrangement. The circuitarrangement 10 further includes a capacitive network 20 having a firstand a second circuit node N1, N2 that are capacitively coupled to theinput 11, and therefore to the output 103 of the half-bridge, and havinga terminal for a reference potential V_(REF). The circuit arrangement 10further includes a recharging circuit 30 for the capacitive network 20,the re-charging circuit including at least one switching element 32.This recharging circuit is adapted during the switched-on phase of oneof the switching elements 101, 102 of the half-bridge to adjustelectrical potentials V_(N1), V_(N2) at the first and the second circuitnode N1, N2 that are different from the reference potential V_(REF). Inthe examples that will first be explained in the following theseelectrical potentials V_(N1), V_(N2) are selected such that they are notonly different from the reference potential V_(REF), but are alsodifferent from one another. However, this additional condition is notmandatory, as will be explained in the following.

The first and the second circuit nodes N1, N2 of the capacitive network20 are connected to a comparator arrangement that is adapted todetermine during the dead time a time difference between those times atwhich the electrical potentials at the first and the second nodes N1, N2each assume a given potential value. In a manner that will be explainedin the following this time difference is a measure for the timevariation of the output voltage. This time variation of the outputvoltage V_(HB) in turn determines the dead time that is at leastrequired to provide for a complete recharging of the output 103 duringthe dead time.

The operating principle of the circuit arrangement illustrated in FIG. 3will become apparent with the help of timing diagrams, that areillustrated in FIG. 4, of the output voltage V_(HB) and with the help ofelectrical potentials V_(N1), V_(N2) of the circuit nodes N1, N2 in thecapacitive network.

Times t1 ₁ and t4 ₁ in FIG. 4 denote a time period during which theoutput voltage V_(HB) assumes an upper voltage level. If switchingdelays of the switching elements 101, 102 are neglected, these are thetimes between which the upper switching element 101 is switched on.During this switched-on period of the first switching element 101, 102the first and the second circuit nodes N1, N2 are charged to electricalpotentials that are different relating to the reference potentialV_(REF). The level of the reference potential V_(REF) is illustrated indash-dotted lines in FIG. 4. For the following explanation it may beassumed that the explained potentials or voltages, respectively, areeach related to the second supply potential or reference potential GND,respectively.

If the output voltage V_(HB) changes starting with time t4, due to thecapacitive coupling of the circuit nodes N1, N2 to the output 103 theelectrical potentials V_(N1), V_(N2) of these circuit nodes also change.Changes with time of these electrical potentials V_(N1), V_(N2) at thefirst and second circuit nodes N1, N2 are identical in the example asillustrated. Due to the different values these electrical potentialsV_(N1), V_(N2) have before a change of the output voltage V_(HB) theelectrical potentials V_(N1), V_(N2) of the two circuit nodes N1, N2reach a given potential value which is for example the potential valueV_(REF) of the reference voltage, at different times. This timedifference Δt is dependent on the difference of the potentials V_(N1),V_(N2) of the two circuit nodes N1, N2, and is dependent on thevariation of the output voltage V_(HB) with time.

The electrical potentials V_(N1), V_(N2) of the two circuit nodes N1, N2are adjusted by the recharging circuit 30 during the switched-on phasesone of the switching elements—i.e., during the switched-on phase of thefirst switching element 101, 102 between times t1 ₁, t4 ₁, forexample—such that they are known or that the differences V_(N1)−V_(REF)or V_(N2)−V_(REF) between these electrical potentials and the referencepotential, respectively are known, and are in one embodimentproportional to the amplitude of the output voltage V_(HB). Knowing thisrelation between the electrical potentials V_(N1), V_(N2) at the firstand second circuit nodes N1, N2, a time variation of the output voltageV_(HB) related to its amplitude may directly be derived from the timedifference Δt. The ratio between the variation of the output voltageV_(HB) with time and the amplitude of the output voltage is a directmeasure for the change with time.

The mentioned proportionality between the differences V_(N1)−V_(REF) orV_(N2)−V_(REF), respectively, that settle in and the amplitude of theoutput voltage V_(HB) means, that these differences change proportionalto the amplitude of the output voltage V_(HB), if the amplitude changes.The amplitude of the output voltage V_(HB) changes, for example, if theinput voltage Vin changes. In this connection it may be assumed thatthose changes occur slowly as compared to the switching frequency of theswitching elements 101, 102.

For explanation purposes it may be assumed that the capacitive couplingbetween the first and second circuit nodes N1, N2 and the output 103 isconstant, and that the potentials V_(N1), V_(N2) at these nodes changelinearly with the output voltage V_(HB). Changes of the potentialsV_(N1), V_(N2) with time are, therefore, proportional to changes of theoutput voltage V_(HB) with time. If one considers additionally theproportionality between the differences V_(N1)−V_(REF) orV_(N1)−V_(REF), respectively, that settle in and the amplitude of theoutput voltage V_(HB), then the time difference Δt is inverselyproportional to the change of the output voltage V_(HB) over time,namely independent of how large the changes of the potentials at thefirst and the second node N1, N2 really are. The only prerequisite isthat the time difference Δt is not zero. This prerequisite can beobtained in that the differences V_(N1)−V_(REF) or V_(N2)−V_(REF),respectively, are set to different values, and that the potentialsV_(N1), V_(N2) with a change of the output voltage V_(HB) change in thesame manner, this being the case for the circuit arrangement explainedwith reference to FIGS. 5 and 8 in the following. Alternatively, thedifferences V_(N1)−V_(REF) or V_(N2)−V_(REF), respectively, may beadjusted to identical values (not zero). In this case the circuit nodesN1, N2 are coupled to the output 103 in such a way that variations withtime of the electrical potentials V_(N1), V_(N2) are different with achange of the output voltage V_(HB).

If the proportionality between the differences V_(N1)−V_(REF) orV_(N2)−V_(REF), respectively, and the amplitude of the output voltageV_(HB) due to parasitic effects is not exactly given, than the circuitfunctions as good as the proportionality is adjusted. If the adaptivelyadjusted dead time is somewhat longer than the recharging time, thensuch non-proportionality can be tolerated.

The comparator arrangement 40 determines this time difference Δt andprovides a dead time signal S10 that is dependent on this timedifference. This dead time signal S10 can be provided to a drive circuit(60 in FIG. 1) for generating the drive signals S1, S2. Dead time signalS10 either provides a measure for the dead time that is to be adjustedbetween the switched-on phases of the two switching elements 101, 102,or represents an enable signal that defines times at which the switchingelement to be switched-on next can be switched-on.

In an equivalent manner as during this switched-on phase of the firstswitching element 101 the change over circuit 30 during the switched-onphase of the second switching element 102 adjusts different potentialsV_(N1), V_(N2) at the first and second circuit nodes N1, N2. In FIG. 4t4 ₂ denotes a time from which the output voltage V_(HB) increases afterswitching off the second switching element 102. Due to the capacitivecoupling of the two circuit nodes N1, N2 to the output 103 theelectrical potentials V_(N1), V_(N2) of these circuit nodes increasewith the same slope as the output voltage V_(HB), with a time differenceΔt between times at which these electrical potentials V_(N1), V_(N2)assume a given potential, such as reference potential V_(REF), directlyrepresents a measure for the change of the output voltage V_(HB) overtime.

FIG. 5 by way of an electrical circuit diagram illustrates a firstexample of the first capacitive network 20 and the recharging circuit30.

In the example as illustrated a capacitive network 20 includes acapacitive voltage divider having a first and a second capacitance 21,22 that are coupled between the input 11 of the circuit arrangement 10or the output 103 of the half-bridge, respectively, and a furtherreference potential. In the example as illustrated the further referencepotential is the second reference potential, so that the output voltageV_(HB) is the voltage across the capacitive voltage divider 21, 20, 22.The first circuit node N1 of the capacitive network is formed by a tapof the capacitive voltage divider, the tap being a circuit node that iscommon to the two capacitances 21, 22 of the voltage divider.

The capacitive network 20 further includes a third capacitance that iscoupled to the tap of the capacitive voltage divider 21, 22. The secondcircuit node N2 is formed by a terminal facing away from the tap of thethird capacitance 23. In this capacitive network 20 the first circuitnode N1 via first capacitance 21 of the voltage divider is capacitivelycoupled to the output 103 of the half-bridge, and the second circuitnode N2 via this series circuit with the first capacitance 21 and thethird capacitance 23 is capacitively coupled to the output 103. Thepotential difference between the electrical potentials V_(N1) of thefirst circuit node N1 and the electrical potential V_(N2) of the secondcircuit node N2 equals the voltage across the third capacitance 23.

The recharging circuit 30 of this circuit arrangement 10 includes afourth capacitance 23 that is connected between the terminal for thefurther reference potential GND and a first switching element 32 of therecharging circuit 30. The first switching element 32 is a selectorswitching element that is adapted to connect the fourth capacitance 33selectively to the second circuit node N2 or to the terminal for thereference potential V_(REF). In the circuit arrangement as illustratedthe reference potential is generated by a reference voltage source 13that is coupled between the terminal for the further referencepotential, i.e., the second supply potential in the example, and theterminal for the reference potential V_(REF). The voltage generated bythe reference voltage source 13 will be referred to as reference voltagein the following. The re-charging circuit 30 further includes a secondswitch 34 that is connected parallel with the third capacitance 23 ofthe capacitive network 20.

The operating principle of the circuit arrangement 10 illustrated inFIG. 5 will now be explained with reference to FIG. 6, in which timingdiagrams of the output voltage V_(HB), of the electrical potentialV_(N1) of the first circuit node N1, and the electrical potential V_(N2)of the second circuit node N2 are illustrated. Further, the drivesignals Φ1, Φ2 for the first and second switch 32, 34 of the change overcircuit are illustrated in FIG. 6. For explanation purposes thesesignals are examined during a complete drive cycle of the half-bridge,the drive cycle beginning at time t0 ₁, at which the half-bridge voltageV_(HB) increases, and ending at time t4 ₁, at which the half-bridgevoltage V_(HB) again starts to increase. During the increase of theoutput voltage V_(HB), the first switch 32 is in a first switchingposition at which the fourth capacitance 33 is connected to thereference potential V_(REF) of voltage source 31; this switchingposition is represented in FIG. 6 by a low-level of drive signal Φ1.During the increase of the output voltage V_(HB) second switch 34 isopen, which is represented by a low-level of the drive voltage signalΦ2. Thus, during the increase of the output voltage V_(HB) the first andthe second circuit nodes N1, N2 are only coupled to the input terminal 1and the output 103 of the half-bridge, respectively, namely capacitivelyvia the first capacitance 21 and the series circuit with the firstcapacitance 21 and the second capacitance 23, respectively. Theelectrical potentials at these circuit nodes N1, N2 thereforeimmediately change with a change of the output voltage V_(HB) over time,with changes with time dV_(N1)/dt and dV_(N2)/dt of the first and secondpotential V_(N1), V_(N2) being identical for the circuit according toFIG. 5. It therefore applies:

$\begin{matrix}{\frac{\mathbb{d}{VN}_{1}}{\mathbb{d}t} = {\frac{\mathbb{d}{VN}_{2}}{\mathbb{d}t}.}} & (1)\end{matrix}$

These changes with time of the first and second potentials V_(N1),V_(N2) are directly related to a change with time of the output voltageV_(HB) via the divider ratio of the capacitive voltage divider 21, 22,where it applies:

$\begin{matrix}{\frac{\mathbb{d}{VN}_{1}}{\mathbb{d}t} = {\frac{\mathbb{d}{VN}_{2}}{\mathbb{d}t} = {\frac{\mathbb{d}V_{HB}}{\mathbb{d}t} \cdot {\frac{C\; 21}{{C\; 21} + {C22}}.}}}} & (2)\end{matrix}$

In this connection C21 and C22 are capacitance values of the first andsecond capacitances of the voltage divider, which will be referred to asfirst and second capacitance value in the following. Referring toequation (2) for a given change with time of the output voltage V_(HB)the changes with time of the first and second potentials V_(N1), V_(N2)are the larger, the larger the first capacitance value C21 is comparedto the second capacitance value C22. By a suitable choice of therelationship between these two capacitance values C21, C22 the voltagechange of the first and second potentials V_(N1), V_(N2) can beadjusted. The input voltage V_(in) of the half-bridge, dependent on theload Z that is to be driven, may amount up to several hundred volts, sothat also the voltage change of the output voltage may amount up toseveral hundred volts. A usual drive of a half-bridge for driving a lampballast having a discharge lamp amounts to about 400V.

With a suitable choice of the first and second capacitance values C21,C22 a large voltage change at output 103 is transferred to acorrespondingly lower voltage change of the first and second potentialsV_(N1), V_(N2) through the capacitive voltage divider 21, 22. Throughthe choice of the capacitance values C21, C22, for example, this voltagechange may be adjusted to amount only some volts, so that for evaluatingthese potentials V_(N1), V_(N2) no high-voltage components are required.In this case only the first capacitance needs to have a voltage blockingcapability of up to several hundred volts. One way of realizing suchcapacitance will be explained hereinbelow.

In FIG. 6 t1 ₁ denotes a time at which the output voltage V_(HB) reachesits upper voltage level. For explanation purposes it may be assumed thatthe first switching element 101 assumes an on-level at this time andstays at this on-level till a later time t4 ₁. Possible delays inswitching on the switching element are not taken into account in theillustration in FIG. 6, because they are not relevant for explaining thebasic operating principle of the circuit arrangement according to FIG.5. During this on-period of the first switching element 101 first thesecond switch 34 of the recharging circuit 30 is switched on in order todischarge the third capacitance 23, and subsequently the first switch 32is switched into the second switching position in order to connect thefourth capacitance 33 to circuit node N2. In FIG. 6 t2 ₁ denotes a timeat which the second switch 34 is closed, and t3 ₁ denotes a time atwhich this switch is again opened. During a time period between time t3₁ and a later time t4 ₁ the first switch 32 is in the second position.In this connection it should be mentioned that the switching periodsduring which the second switch 34 is closed and during which the firstswitch 32 is in the second switching position are not necessarilysubsequent, but may be distant to one another over time. Further, thetime period during which the first switch 32 is in the second switchingposition may already end before time t4 ₁ from which the output voltageV_(HB) starts to decrease.

The explained driving of the first and second switches 32, 34 during theswitched-on phase t1 ₁-t4 ₁ of the first switching element 101 serves toadjust the electrical potentials V_(N1), V_(N2) at the first and secondcircuit nodes N1, N2 to different potential values related to thereference potential V_(REF) or related to the further referencepotential GND.

As will be explained hereinbelow the second potential V_(N1) increaseswith increasing output voltage V_(HB) to a higher final value than thefirst potential V_(N1). In this connection “final value” denotes thevoltage value the first and second potentials V_(N1), V_(N2) have whenthe output voltage V_(HB) reaches its upper voltage level. If the secondswitching element 34 is closed during time period t2 ₁-t3 ₁, the thirdcapacitance 23, that has been charged before, is discharged, and thesecond potential V_(N2) decreases to the value of the first potentialV_(N1) at first.

As it will also be explained the first electrical potential V_(N1)during the switched-on phase of the first switching element 101 andduring the time period, during which the output voltage V_(HP) assumesits upper voltage level, lies above the reference potential V_(REF). Ifthe first switch 24 is opened, and the second switch 32 is in the secondswitching position, the second capacitance 22 is discharged for thebenefit of the third capacitance 24 and the fourth capacitance 33. Thisresults in that the first electrical potential V_(N1) decreases, and inthat also the second electrical potential V_(N2) decreases, namely untilthe sum of the voltages across the third capacitance 23 and the secondelectrical potential V_(N2) equals the first electrical potentialV_(N1).

It can be illustrated that the third capacitance 23 is charged to avoltage V23 during the time period t3 ₁-t4 ₁ for which applies:

$\begin{matrix}{{V\; 23\left( {t\; 4_{1}} \right)} = {\left\lbrack \frac{{V_{N\; 1}\left( {t\; 3_{1}} \right)} - V_{REF}}{\frac{1}{C\; 22} + \frac{1}{C\; 23} + \frac{1}{C\; 33}} \right\rbrack \cdot {\frac{1}{C\; 23}.}}} & (3)\end{matrix}$

In this connection V_(N1) denotes the electrical potential at the firstnode N1 at time t3 ₁, V_(REF) denotes the reference voltage, C22 denotesthe capacitance value of the second capacitance, C23 the capacitancevalue of the third capacitance 23, and C33 the capacitance value of thefourth capacitance 33. Equation (3) is valid under the assumption thatthe capacitance value C21 of the first capacitance 21 is almostnegligible compared to the capacitance value C22 of the secondcapacitance 22, i.e., if, for example, the capacitance value C21 of thefirst capacitance 21 is smaller than the capacitance value C22 of thesecond capacitance 22 for a factor 50 or more, or in one embodiment fora factor 100 or more.

For the first electrical potential V_(N1) at the end of the switchingtime of the first switch 32, i.e., at time t4 ₁, the following appliesaccordingly and under the same assumption:

$\begin{matrix}{{V_{N\; 1}\left( {t\; 4_{1}} \right)} = {{V_{N\; 1}\left( {t\; 3_{1}} \right)} - {\frac{1}{C\; 22} \cdot {\left\lbrack \frac{{V_{N\; 1}\left( {t\; 3_{1}} \right)} - V_{REF}}{\frac{1}{C\; 22} + \frac{1}{C\; 23} + \frac{1}{C\; 33}} \right\rbrack.}}}} & (4)\end{matrix}$

In this connection V_(N1)(t4 ₁) denotes the first potential V_(N1) attime t4 ₁, V_(N1)(t3 ₁) denotes the first potential V_(N1) at time t3 ₁,i.e., before discharging the second capacitance 22. For the secondelectrical potential V_(N2) at time t4 ₁ applies:V _(N2)(t4₁)=V _(N1)(t4₁)−V23(t4₁)   (5).

Considering equations (3) and (4) it is obvious that the first andsecond electrical potentials at time t4 ₁ have a fixed relationship witheach other that is only dependent on the capacitance values of thesecond, third and fourth capacitances 22, 23, 33, the reference voltageV_(REF) and the first electrical potential V_(N1)(t3 ₁) at time t3 ₁.

From time t4 ₁ the second switch 34 is still open, and the first switch32 is again in the first switching position, so that the first and thesecond circuit nodes N1, N2 are only capacitively coupled to the output103. If the output voltage V_(HB) decreases starting with time t4 ₁,then the electrical potentials at the first and second circuit nodes N1,N2 change in accordance with equations (1) and (2), that have beenexplained hereinabove. In FIG. 6 t5 ₁ denotes a time at which the secondelectrical potential V_(N2) reaches the reference potential V_(REF), andt6 ₁ denotes a time at which the first electrical potential V_(N1)reaches the reference potential V_(REF). Δt₁ denotes a time differencebetween these two times t5 ₁, t6 ₂, Δt₁ being a direct measure for thechange with time of the output voltage V_(HB), which will be explainedfurther below.

In FIG. 6 t1 ₂ denotes a time at which the output voltage V_(HB) hasdecreased to its lower voltage level. From this time during the negativehalf-cycle of the output voltage V_(HB) the same drive cycle of thefirst and second switches 32, 34 starts, as it has been explained beforefor the time period t1 ₁ to t4 ₁ during the positive half-cycle of theoutput voltage V_(HB). Same times during the negative half-cycle aredenoted with the same reference characters as the corresponding timesduring the positive half-cycle, these reference characters being onlydifferent in the subscript indices, with index “1” denoting the timesduring the positive half-cycle, and the index “2” denoting thecorresponding times during the negative half-cycle. During a time periodt2 ₂ to t3 ₂ the second switch 34 is closed in order to discharge thethird capacitance 23. The second electrical potential V_(N2) adaptsitself to the first electrical potential V_(N1), which means during thenegative half-cycle that the second electrical potential V_(N2)increases. During the time period t3 ₂ to t4 ₂, during which the firstswitch 32 is in the second switching position in order to connected thefourth capacitance to the second circuit node N2, the second capacitance22 is charged during the negative half-cycle by the fourth capacitance33, that has been charged to the reference voltage V_(REF) before.Equations (3) to (5) for the voltage V23 across the third capacitance23, as well for the first and second electrical potential V_(N1), V_(N2)at time t4 ₂, apply accordingly, where in these equations time t3 ₁ isto be replaced by t3 ₂, and t4 ₁ is to be replaced by t4 ₂.

In FIG. 6 t4 ₂ denotes a time at which the output voltage V_(HB), andthus the first and second electrical potentials V_(N1), V_(N2) increase.At a time t5 ₂ the second electrical potential V_(N2) reaches thereference potential V_(REF), and at a time t6 ₂ the first potentialV_(N1) reaches the reference voltage V_(REF). In FIG. 6 Δt₂ denotes thetime difference between these times t5 ₂, t6 ₂.

If the input voltage of the half-bridge (Vin in FIG. 1) does not changeover the time, then the voltage change during the transfer from thelower voltage level to the upper voltage level and during the transferfrom the upper voltage level to the lower voltage level is identical. Insteady-state, i.e., after a sufficient number of switching periods withthe same voltage change of the output voltage V_(HB), the timingdiagrams of the first and second potentials V_(N1), V_(N2) aresymmetrically related to the reference potential V_(REF). Thus, thefirst potential V_(N1) before a falling edge of the output voltageV_(HB) has the same amplitude related to the reference potential V_(REF)as before a rising edge of the output voltage V_(HB), the absolute valueof the difference between the first potential V_(N1) and the referencepotential V_(REF) is thus identical before a rising edge and before afalling edge of the output voltage V_(HB), i.e., at times t4 ₁, t4 ₂. Inthe explanation hereinabove these amplitudes of the first potentialV_(N1) before the falling edge and the rising edge, respectively, of theoutput voltage V_(HB) are referred to as V_(N1)(t4 ₁) and V_(N1)(t4 ₂).It therefore applies:|V _(N1)(t4₁)−V _(REF) |=|V _(N1)(t4₂)−V _(REF)|  (6a).

Accordingly the amplitude of the second potential V_(N2) related to thereference potential V_(REF) before a falling edge and before a risingedge of the output voltage V_(HB) is identical, so that the followingapplies accordingly:|V _(N2)(t4₁)−V _(REF) |=|V _(N2)(t4₂)−V _(REF)|  (6b).

If the input voltage Vin is unchanged, then the absolute changes of thefirst and second potentials V_(N1), V_(N2) are identical during therising edge and during the falling edge of the output voltage V_(HB).This change with time during the dead time, which will be referred to assignal change in the following, according to equation (2) is dependenton the signal change of the output voltage V_(HB) via the divider ratioof the capacitive voltage divider. In steady-state of the circuitarrangement the time differences Δt₁, Δt₂ are, however, not dependent onthe capacitive voltage divider ratio, but are only dependent on thecapacitances within the capacitive network 20 and recharging circuit 30.As it will be explained in the following, this allows for a use of acapacitive component as the first capacitance that has no specificrequirements concerning precision of the capacitance value. Thus, acapacitive component may be used as the first capacitance 21 that, dueto the production process, has high tolerances of its capacitance value.

The independence of the time difference At of the capacitance value C21of the first capacitance applies under the assumption that thecapacitance value C21 of the first capacitance C21 is negligiblecompared to the capacitance value C22 of the second capacitance 22. Inthis case the amplitude or the signal change, respectively, of theoutput voltage V_(HB) is large compared to the signal changes of thefirst and second potentials V_(N1), V_(N2). The product of the signalchange of the output voltage V_(HB) and the capacitance value C21 of thefirst capacitance then equals the amount of charge that during eachrecharging flows into the second capacitor 22 or from this. A change ofthe capacitance value C21 of the first capacitance therefore has thesame influence as a change of the signal change of the output voltageV_(HB). If, as it has been explained before, the voltage differencesV_(N1)−V_(REF) and V_(N2)−V_(REF) respectively are proportional to theamplitude of the output voltage V_(HB), then a change of the firstcapacitance C21, like a change of a signal change, only effects scalingthe timing diagrams of the first and second potentials V_(N1), V_(N2)related to the reference voltage V_(REF) as a virtual zero-line. Thewaveforms or their time dependencies do not change. Since V_(REF) issimultaneously the comparator threshold for determining the timedifference Δt, a scaling does not affect these “zero-crossings” t5, t6related to V_(REF).

The symmetry of the first and second potentials V_(N1), V_(N2) to thereference voltage V_(REF) is achieved in the capacitive network 20illustrated in FIG. 5 in that the second circuit node N2, and via thethird capacitance 23 also the first circuit node N1, is connected to thefourth capacitance 33, that has been charged to the reference voltageV_(REF) before, during each switched-on phase of a switching element101, 102. During the switched-on phase of the first switching element101 (positive half-cycle of the output voltage V_(HB)) the secondcapacitance 22 is partly discharged for the benefit of the fourthcapacitance 33, and during the switched-on phase of the second switchingelement 102 (negative half-cycle of the output voltage V_(HB)) thesecond capacitance is charged on account of the fourth capacitance 23.The capacitive network 20 is in its steady state—with unchanged inputvoltage Vin over time—, if the charge that flows from the secondcapacitance 22 during the switched-on phase of the first switchingelement 21 equals the charge that flows back into the second capacitance22 during the switched-on phase of the second switching element 102 fromthe fourth capacitance 23. A transient phase until reaching suchsteady-state will now be explained with reference to FIG. 7.

FIG. 7 illustrates exemplary timing diagrams of the output voltageV_(HB) of the first potential V_(N1). For the purpose of explanation itmay be assumed that the first capacitance 22 is not charged before afirst increase of the output voltage V_(HB), i.e., that its charge attime t0 ₀₁ is zero. If the output voltage V_(HB) increases to its uppervoltage value then the first potential V_(N1) at time t1 ₀₁, at whichthe output voltage V_(HB) for the first time reaches the upper voltagelevel, is directly dependent on the output voltage V_(HB) via thedivider ratio of the capacitive voltage divider. In FIG. 7 t3 ₀₁ denotesa time at which the first switch 32 is switched into the secondswitching position in order to connect the fourth capacitance 33 to thesecond circuit node N2, so that the second capacitance 22 is discharged.Since the second capacitance 22 at the beginning of the transient phaseis less charged then in the steady-state, from time t3 the secondcapacitance 22 is first charged via the third capacitance 33, so thatthe potential at the first node N1 increases. If from time t4 the outputvoltage V_(HB) decreases, then the second capacitance 22 is notcompletely charged anymore, i.e., the first potential V_(N1) does notdecrease to the further reference potential GND anymore. During thenegative half-cycle of the output voltage V_(HB) the second capacitance22 at closed second switch 32 is also charged by the fourth capacitance33, namely from time t2 ₀₂ according to FIG. 7. The transient phase hascome to an and, when the second capacitance 22 after a number of drivecycles is charged to such an extent that during the positive half-cycleof the output voltage V_(HB) the same amount of charge ΔQ1 flows fromthe second capacitance 22 at closed second switch 32 into the fourthcapacitance 23, as charge ΔQ2 flows during the negative half-cycle ofthe output voltage V_(HB) at closed second switch 32 from the fourthcapacitance 33 into the second capacitance 22. It therefore applies:−ΔQ1=ΔQ2.

FIG. 8 illustrates a further example of the recharging circuit 30. Thefourth capacitance 33 of this recharging circuit 30 is connected betweenthe terminal 12 for the reference potential V_(REF) and the first switch32, the first switch 32 being adapted to alternatively connect theterminal of the fourth capacitance 33—which in the example faces awayfrom the reference potential terminal 12—to the reference potentialterminal 12 or the second circuit node N2. In a manner not illustratedin detail in the circuit according to FIG. 8 the fourth capacitance 33,the first switch 32, and the second circuit node N2 could also beinterconnected according to the example of FIG. 5, which means so thatthe third capacitance with one terminal permanently lies on the furtherreference potential, and that the other terminals by the first switch 32can be switched between the terminal 12 for the reference potentialV_(REF) and the second terminal N2. In a corresponding manner, in thecircuit according to FIG. 5 fourth capacitance 33 the first switch 32,and the second circuit node N2 could also be interconnected according tothe example illustrated in FIG. 8.

The recharging circuit 30 further includes a bridge circuit having fourbridge switches 37 ₁, 37 ₂, 38 ₁, 38 ₂ and a fifth capacitance 34. Thisbridge circuit is connected between the first circuit node N1 and theterminal 12 for the second reference potential V_(REF). The four bridgeswitches and the fifth capacitance 34 are interconnected such, that afirst switch pair with a first bridge switch 37 ₁, and a second bridgeswitch 37 ₂ switches the fifth capacitance 34, so as to have a firstpolarity, between the first circuit node N1 and the reference potentialterminal 12, when these switches 37 ₁, 37 ₂ are closed, and that asecond switch pair with a third and a fourth bridge switch 38 ₁, 38 ₂switches the fifth capacitance 34, so as to have a second polarity,between the first circuit node N1 and the reference potential terminal12, when the switches of this switch pair are closed. Thus, when theswitches 37 ₁, 37 ₂ of the first switch pair are closed, a firstterminal of the fifth capacitance 34 is connected to the first circuitnode N1, and a second circuit node of the fourth capacitance 34 isconnected to the reference potential terminal 12. With conductingswitches 38 ₁, 38 ₂ of the second switch pair, in a correspondingmanner, the second terminal of the fifth capacitance 34 is connected tothe first circuit node N1, and the first terminal of the fifthcapacitance 34 is connected to the reference potential terminal 12.Optionally a sixth capacitance 35 is connected between the firstterminal of the fifth capacitance 34 and reference potential terminal12, and between the second terminal of the fifth capacitance 34 and thereference potential terminal 12 a seventh capacitance 36 is connected.

The operating principle of the circuit arrangement illustrated in FIG. 8will become apparent with the help of timing diagrams of the outputvoltage V_(HB), and the first and second potentials V_(N1), V_(N2) atthe first and second circuit nodes N1, N2, the timing diagrams of whichare illustrated in FIG. 9 for the steady-state. Illustrated in FIG. 9are the timing diagrams of the drive signals of the individual switches.Φ1 denotes the drive signal of the first switch 32. In the exampleillustrated a low signal level of this drive signal Φ1 represents aswitching state in which the fourth capacitance 33 is connected to thesecond reference potential terminal 12, so that the fourth capacitance33 is short-circuited. This switching state will be referred to as firstswitching state in the following. An upper signal level of the drivesignal Φ1 represents a switching state of the first switch 32 in whichthe fourth capacitance 33 is connected to the second circuit node N2.This switching state will be referred to as second switching state inthe following.

In FIG. 9 Φ3 denotes the common drive signal of the bridge switches 37₁, 37 ₂ of the first switch pair, and Φ4 denotes the common drive signalfor the bridge switches 38 ₁, 38 ₂ of the second switch pair. In theexample illustrated the bridge switches are closed each time, theirdrive signal Φ3 or Φ4 assumes an upper signal level, and the switchesare opened each time, their drive signal Φ3 or Φ4 assume a lower signallevel. For a better understanding the first and second drive signals S1,S2 of the two half-bridge switches 101, 102 are also illustrated in FIG.9.

In correspondence with the timing diagram in FIG. 6 t1 ₁ in FIG. 9denotes a time at which the output voltage V_(HB) reaches its maximumvalue during its positive half-cycle. Till this time all bridge switches37 ₁, 37 ₂, 38 ₁, 38 ₂ are open, and the fourth capacitance 33 isshort-circuited via the first switch 32. At a later time t2 ₁ during thepositive half-cycle the bridge switches 37 ₁, 37 ₂ of the second bridgeswitch pair are closed, and the first switch 32 is switched into thesecond switching position, in order to connect the fourth capacitance 32between the reference potential terminal 12 and the second circuit nodeN2. Through this the second capacitance 32 is discharged via optionalsixth capacitance 35, so that the first potential V_(N1) decreases. Inthe steady-state the charge in the capacitance 34 does not change.Further, the fourth capacitance 33 is discharged, so that the secondelectrical potential V_(N2) also decreases. Through this, at the firstand second nodes N1, N2 electrical potentials settle in that aredifferent related to the reference potential V_(REF). To what extent thesecond potential V_(N2) decreases during this time period, i.e., to whatextent this potential approaches the reference potential V_(REF), isdependent on the capacitive capacitive voltage divider of the dividerratio formed by the third and fourth capacitance 23, 33. In FIG. 9 t3 ₁denotes a time at which the recharging process at the first and secondcircuit nodes N1, N2 has come to an end, and at which the bridgeswitches 37 ₁, 37 ₂ are opened and the first switch 32 is again switchedinto the first switching state in order to interrupt the connectionbetween the second circuit node N2 and the fourth capacitance 33. For achange in the second potential V_(N2) between times t2 ₁ and t3 ₁ itapplies:

$\begin{matrix}{{\Delta\; V_{N\; 2}} = {{{V_{N\; 2}\left( {t\; 2_{1}} \right)} - {V_{N\; 2}\left( {t\; 3_{1}} \right)}} = {\frac{C\; 33}{\frac{C\;{22 \cdot C}\; 23}{{C\; 22} + {C\; 23}} + {C\; 33}} \cdot {\left\lbrack {{V_{N\; 2}\left( {t\; 2_{1}} \right)} - V_{REF}} \right\rbrack.}}}} & (7)\end{matrix}$

In this connection C23 denotes the capacitance value of the thirdcapacitance 23, and C33 denotes the capacitance value of the fourthcapacitance 33. The electrical potential VN2 decreases between times t2₁ and t3 ₁ the more, the larger the capacitance value C33 of the fourthcapacitance 33 is compared with the capacitance of the series circuitwith the second and third capacitances 22, 23. The ratio C33/C23 of thecapacitance values of the third and fourth capacitances 23, 33 is, forexample, between two and ten.

The change of the electrical potential V_(N1) between times t2 ₁ and t3₁ is significantly determined by the ratio between the capacitancevalues C22 of the second capacitance and of the series circuit with thethird and fourth capacitances 23, 33. These ratios are similar to theratios in the circuit according to FIG. 5, with the difference that thethird capacitance 23 is not discharged in between. From times t2 ₁ at t2₂ C23 is charged oppositely. The amount of charge that is required forthis is twice as it would be, if third capacitance 23 would bedischarged in between—such as it is the case in the circuit according toFIG. 5. The capacitance values of the optional sixth and seventhcapacitances 35, 36 may be significantly smaller than the capacitancevalue of the fifth capacitance. The capacitance ratio between thecapacitance value C35, C36 of one of these sixth and seventhcapacitances and the capacitance value C34 of the fifth capacitance 34is, for example, between ⅕ and ⅙. The sixth and seventh capacitances 35,36 may, for example, be formed by line capacitances.

The capacitance value C34 of the fifth capacitance 34 may be in the sameorder of magnitude than the capacitance value C22 of the secondcapacitance 22. The ratio C34/C32 of this capacitance values is, forexample, between 0,2 and 5.

By recharging processes of the first and second circuit nodes N1, N2between times t2 ₁, t3 ₁ electrical potentials V_(N1), V_(N2) that arerelated to the second reference potential V_(REF) and that are differentfrom one another settle in at the first and second circuit nodes N1, N2.

In FIG. 9 t4 ₁ denotes a time from which the output voltage V_(HB)decreases. The electrical potentials V_(N1), V_(N2) at the first andsecond circuit nodes N1, N2 change in the same manner, namely dependenton the changes with time of the output voltage V_(HB). The changes withtime of the electrical potentials V_(N1), V_(N2) at the first and secondcircuit nodes N1, N2 stand in relationship with the change with time ofthe output voltage V_(HB) according to equation (2).

In FIG. 9 t5 ₁, t6 ₁ denote times at which the first and secondelectrical potential V_(N1), V_(N2) with the falling edge of the outputvoltage V_(HB) reach the reference potential V_(Ref). A time differenceΔt₁ is here a direct measure for the change with time of the outputvoltage V_(HB).

In FIG. 9 t1 ₂ denotes a time at which the output voltage reaches itsmaximum (negative) amplitude value during the negative half-cycle. Afterthis time t1 ₂ the first switch 32 is again switched over at a time t2 ₂in order to connect the third capacitance 33 to the second circuit nodeN2, and the bridge switches 38 ₁, 38 ₂ of the second switch pair areclosed in order to connect the fifth capacitance 34 with reversedpolarity between the first circuit node N1 and the reference potentialterminal 12. Consequently the second capacitance 22 is charged via theseries circuit with the third and fourth capacitances 22, 33, the chargepresent in the third capacitance 33 from the previous half-cyclecontributing to charging the second capacitance 22. Thus, the potentialV_(N2) at the second circuit node N2 increases. In the steady-state thevalue of the first electrical potential V_(N1) at time t3 ₁ related tothe reference potential V_(Ref) is symmetrical to the value of the firstelectrical potential V_(N1) at time t3 ₂. Correspondingly the value ofthe second electrical potential V_(N2) at time t3 ₁ related to thereference potential V_(Ref) is symmetrical to the value of the secondelectric potential V_(N2) at time t3 ₂. The timing diagrams of the firstand second electrical potentials V_(N1), V_(N2) in the steady-state aretherefore symmetrical related to the second reference potential V_(Ref).In the circuit arrangement according to FIG. 8 a steady-state is reachedthe faster, the larger the capacitance value C34 of the fifthcapacitance 34 is compared to the capacitance value C22 of the secondcapacitance 22.

In the scenario illustrated in FIG. 9 the output voltage V_(HB)increases at time t4 ₂, with the electrical potentials V_(N1), V_(N2) atthe first and second circuit nodes increasing in a corresponding manner.In this connection t5 ₂, t6 ₂ denote times at which the first and secondelectrical potentials V_(N1), V_(N2) each reach the value of the secondreference potential V_(Ref). A time difference Δt₂ between these timest5 ₂, t6 ₂ is here a direct measure for the temporal change of theoutput voltage V_(HB). In the steady-state these time differences Δt₁,Δt₂, that are determined for the positive and the negative half-cycle ofthe output voltage V_(HB), are identical, provided that the rechargingtimes during the positive and the negative half-cycles are alsoidentical, which is usually the case in lamp ballasts and LLCconverters.

In connection with the explanation that has been given with reference toFIG. 9 it has been assumed that the time periods during which one of theswitching pairs of the bridge circuit is switched on, and during whichthe first switch 32 connects the third capacitance 33 to the secondcircuit node N2, are identical. In this connection it should be pointedout that this is only an example. The time periods during which theswitches of a switch pair are closed, and the time period during whichthe first switch 32 connects the fourth capacitance 33 to the secondcircuit node N2, could also be timely offset to each other, providedthat these time periods each lie within the on-period of one of thehalf-bridge switches.

The drive signals Φ1-Φ4 explained in connection with FIGS. 6 and 9 candirectly be obtained from the drive signals S1, S2 of the half-bridgeswitches. In the example explained with reference to FIG. 6 the timeperiod, during which the second drive signal Φ2 assumes an upper signallevel, starts, for example, each time after a delay time after an uppersignal level of the first or second drive signal S1, T2. The time periodduring which the second drive signal Φ2 assumes an upper signal levelis, for example, a given fraction of the overall time period, duringwhich the first and second drive signals S1, S2 each assume an uppersignal level. This time period, for example, may be measured during aprevious drive cycle, so that the method also works for changingswitching frequencies of the two switches 101, 102 of the half-bridge.In the example illustrated in FIG. 6 the upper signal level of the firstdrive signal Φ1 starts, whenever the second drive signal Φ2 assumes alower signal level. The time period of the upper signal level of thefirst drive signal Φ1 ends each time the drive signals S1 and S2,respectively, assume the lower signal levels.

Alternatively the upper signal level of the second drive signal Φ2 maybe generated for a fixed time period, this time period being selectedsuch that it is significantly shorter than the shortest possibleon-period of the first and second drive signals S1, S2. This on-periodis, for example, 50% or less of the shortest possible on-period.

In the method illustrated with reference to FIG. 9 the drive signal Φ1for the first switch 32 is, for example, generated in such a manner thatit assumes an on-level after expiration of a delay time after anon-level of the first or second drive signals S1, S2, and that it keepsthe on-level for a given time period. This given time period is, forexample, a fraction of the overall on-period of the first or secondswitching signals S1, S2, or a fixed time period. Alternatively thistime period may end, when the drive signals S1 and S2, respectively,end. The third and fourth drive signals Φ3 and Φ4 are generated such,that Φ3 equals the second drive signal Φ2 during an on-level of thefirst switching signal S1, and that Φ4 equals the second drive signal Φ2during an on-level of the second drive signal S2.

In connection with FIGS. 6 and 9 it should be mentioned that thesefigures are simplified illustrations, because the charging anddischarging processes of the capacitances are illustrated by linearwaveforms instead of the correct exponential waveforms.

In the circuit arrangements illustrated with reference to FIGS. 5 and 8the first and second circuit nodes N1, N2 are coupled to the output 103in such a manner that with a change of the output voltage V_(HB) thepotentials V_(N1), V_(N2) at these two nodes N1, N2 change in anidentical manner. A time difference Δt is here reached due to the factthat the differences V_(N1)−V_(Ref), V_(N2)−V_(Ref), that settle inprior to edges of the output voltage V_(HB), are different from eachother. Different from this the nodes N1, N2 could also be coupled to theoutput 103 in such a manner that the potentials V_(N1), V_(N2) at thesenodes N1, N2 each change in a different manner. The differencesV_(N1)−V_(Ref, V) _(N2)−V_(Ref) could than be set to identical values orto different values before the edges of the output voltage V_(HB), aslong as it is assured that these differences are proportional to theamplitude of the output voltage, and that the changes with time of thepotentials V_(N1), V_(N2) at the first and second nodes N1, N2 areproportional to the changes with time of the output voltages V_(HB).

A capacitive coupling of the first and second circuit nodes N1, N2 tothe output 103, that effects changes with time of the potentials V_(N1),V_(N2),may be obtained in the circuits according to

FIGS. 5 and 8 in that, for example, the capacitive divider ratios aredifferent for the first and second nodes N1, N2. For this a capacitivevoltage divider 21, 22 could be expanded by adding a further voltagedivider capacitance connected in series. The voltage divider obtainedthrough this has two taps, one of which forming the first node N1, andthe other one forming the second node N2. Alternatively the thirdcapacitance 23 could be supplemented to a capacitive voltage divider bya further capacitor to be connected between node N2 and a referencepotential. An example of a drive circuit 60 for generating the first andsecond drive signals, S1, S2 for the two half-bridge switches 101, 102is illustrated in FIG. 10. This drive circuit includes an oscillator 61that is adapted to generate a rectangular oscillator signal S61. Thefrequency of this oscillator signal may, for example, be adjusted by acontrol signal S62 (illustrated in dashed lines) provided to theoscillator 61. The drive circuit further includes a first and a secondflip-flop 62, 63, each having a set input S, a reset input R, and anoutput Q. The first drive signal S1 is available at output Q of thefirst flip-flop 62, and the second drive signal S2 is available atoutput Q of the second flip-flop 63. It is assumed for the purpose ofthe present explanation that the drive signals S1, S2 each assume anon-level, when the corresponding flip-flop 62, 63 is set, and assumes anof-level, when the corresponding flip-flop 62, 63 is reset.

In the drive circuit 60 that is illustrated the two flip-flops 62, 63are set dependent on the oscillator signal S61 and the dead time signalS10, and are reset dependent on the oscillator signal S61. The firstflip-flop 62 is set each time the oscillator signal 61 assumes an uppersignal level and the dead time signal S10 signals an end of the deadtime. The end of the dead time is, for example, signaled by a risingedge of the dead time signal S10. For this the set input 62 of the firstFlip-Flop 62 receives the oscillator signal S61 and the dead time signalS10 via an AND-gate 64. Flip-flop 62 is reset with each falling edge ofthe oscillator signal S61. For this reset input 62 of the firstflip-flop is an inverting input. The second flip-flop 63 is set eachtime the oscillator signal S61 assumes a lower signal level, and thedead time signal S10 signals an end of the dead time. In the exampleillustrated the set input S of the second flip-flop 63 receives theinverted oscillator S61—inverted by an inverter 64, and the dead timesignal S10 that are AND-gated by a second AND gate 66. The secondflip-Flop f3 is reset with each rising edge of the oscillator signal 61.For this the oscillator signal S 61 is directly fed to the reset input Rof the second Flip-Flop 63.

The operating principle of the drive circuit 60 illustrated in FIG. 10becomes apparent by way of FIG. 11, in which exemplary timing diagramsof the oscillator signal 61, the dead time signal S10, as well as thefirst and second drive signals S1, S2 are illustrated. As it can be seenfrom FIG. 11 the first drive signal S1 assumes an on-level afterexpiration of a dead time after a rising edge of the oscillator signalS61, the on-level lasting till the falling edge of the oscillator signalS61. The second drive signal S2 assumes an on-level each time afterexpiration of the dead time after a falling edge of the oscillatorsignal S61, the on-level lasting till the next rising edge of theoscillator signal S61. The dead time is determined by the dead timesignal S10 and, more exactly, by a rising edge of the dead time signalS10 in the example illustrated.

An example of a dead time signal generation circuit 40 for generatingthe dead time signal S10 is illustrated in FIG. 10. This circuitarrangement includes two comparators: A first comparator 41 thatreceives the first potential V_(N1) and the reference potential V_(REF),and that generates a first comparator signal S41 dependent on acomparison of these two potentials; and a second comparator 42 thatreceives the second potential V_(N2) and the reference potentialV_(REF), and that generates a second comparator signal S42 dependent ona comparison of these two potentials. In the example illustrated thefirst and second potentials V_(N1), V_(N2) are each provided to thenon-inverting inputs of the comparators 41, 42, while the secondreference potential V_(REF) is provided to the inverting input of thesecomparators 41, 42. The provision of these signals to the inverting andnone-inverting inputs of the two comparators could also be interchanged.The two comparator signals S41, S42 are provided to an evaluationcircuit 42 that generates the dead time signal S10.

An example of the evaluation circle 43 is illustrated in FIG. 13. Thisevaluation circuit is adapted to determine the dead time after a fallingedge of the first or second drive signals S1, S2 from the timedifference At, this time difference At being represented by the twocomparator for signals S41, S42. In this connection At denotes one ofthe two time differences Δt₁, Δt₂ explained before with reference toFIGS. 6 and 9 that are identical in the steady-state.

The evaluation circuit 43 that is illustrated includes two seriescircuit each having a capacitance 51 ₁, 51 ₂, a switching element 52 ₁,52 ₂, and a current source 53 ₁, 53 ₂. The evaluation circuit furtherincludes a comparator 54 that is adapted to evaluate electrical voltagesacross the capacitances 51 ₁, 51 ₂, and to generate the dead time signalS10 dependent on a comparison of these voltages V51 ₁, V51 ₂. The firstand the second series circuits are adapted to one another in such amanner that the voltage V51 ₁ across capacitance 51 ₁ of the firstserious circuit with closed switch 52 ₁ increases slower than thevoltage V51 ₂ across capacitance 51 ₂ of the second serious circuit withclosed switch 52 ₂. This can be obtained by suitably choosing thecapacitance values of these capacitances 51 ₁, 51 ₂ and suitablychoosing the current sources 53 ₁, 53 ₂, where for changes with timedV51 ₁/dt and dV51 ₂/dt, respectively, of the voltages V51 ₁ and V51 ₂,respectively, across the capacitances 51 ₁, 51 ₂ it applies:

$\begin{matrix}{\frac{{\mathbb{d}V}\; 51_{1}}{\mathbb{d}t} = {\frac{I\; 53_{1}}{C\; 51_{1}}.}} & \left( {8a} \right) \\{\frac{{\mathbb{d}V}\; 51_{2}}{\mathbb{d}t} = {\frac{I\; 53_{2}}{C\; 51_{2}}.}} & \left( {8b} \right)\end{matrix}$

In this connection 153 ₁ and 153 ₂, respectively, denote the currentsprovided by the current sources 53 ₁, 53 ₂, and C51 ₁, C51 ₂ denote thecapacitance values of the capacitance 51 ₁, 51 ₂.

The operating principle of the evaluation circuit illustrated in FIG. 13becomes apparent by way of FIG. 14, in which timing diagrams of some ofthe signals occurring in the evaluation circuit 43 are illustrated.

Switches 52 ₁, 52 ₂ of the two series circuits are driven by drivesignals S52 ₁, S52 ₂. In the evaluation circuit that is illustratedswitched 52 ₁ of the first series circuit is switched on with eachfalling edge of the first or second drive signals S1, S2. For this,drive signal S52 ₁, is, for example generated using a flip-flop 54having an inverting input S that receives from an OR gate 55 a signalobtained by OR gating the first and second drive signals with S1, S2.Referring to FIG. 14 the voltage V51 ₁ across the first capacitance 51 ₁continuously increases from the falling edge of the first or seconddrive signals S1, S2, this increase over time being determined byequation (8 a).

Switch 52 ₂ of the second series circuit is switched on only during timeperiod Δt, in order to charge the capacitance 51 ₂ of the second seriescircuit. The drive signal S52 ₂ for this switch 52 ₂ is, for example,generated from the first and second comparator signals S41, S42 by anXOR gate 56.

The dead time T_(D) starts with the falling edge of the first or seconddrive signals S1, S2, and ends when the voltage V51 ₁ across thecapacitance 51 ₁ has increased to the value of the voltage V51 ₂ acrossfaster charged capacitance 51 ₂. After reaching the dead time T_(D) thecapacitances 51 ₁, 51 ₂ are discharged via switches 57 ₁, 57 ₂ connectedin parallel. These switches 57 ₁, 57 ₂ are, for example, driven by drivesignals (not illustrated) that may correspond to the first or seconddrive signals Φ1 or Φ2.

The dead time T_(D) that is determined using the explained dead timesignal generation circuit, is directly proportional to the timedifference Δt, that is given by the two comparator signals S41, S42. Forthis dead time T_(D) it applies, independent of the time difference Δt:

$\begin{matrix}{T_{D} = {{\frac{I\; 53_{2}}{C\; 53_{2}} \cdot \frac{C\; 53_{1}}{I\; 53_{1}} \cdot \Delta}\;{t.}}} & (9)\end{matrix}$

Thus, dead time T_(D) besides the time difference is only dependent onthe dimensioning of the two series circuits.

It should be mentioned that in FIG. 14 determining the dead time isillustrated for the time period of a falling edge of the output voltageV_(HB), i.e., for the time period after a falling edge of the firstdrive signal S1. Determining the dead time for the time period during arising edge of the output voltage V_(HB), i.e., after a falling edge ofthe second drive signal S2 works, of course, correspondingly.

Optionally the evaluation circuit 40 includes a further evaluation unit44 that is adapted to determine if the zero crossings of the first andsecond potentials V_(N1), V_(N2) occur within a given time period afterbeginning of an edge of the output voltage V_(HB), i.e., after a fallingedge of the first or second drive signals S1, S2. For this theevaluation unit 44 receives the first and the second comparator signalsS41, S42 and a further signal, the further signal indicating fallingedges of the first and second drive signals S1, S2. This further signalis, for example, signal S52 ₁ according to FIG. 13.

The evaluation unit 44 is adapted to generate two error signals S44 ₁,S44 ₂: The first error signal S44 ₁ represents a first error scenariofor which the two zero crossings, i.e., the zero crossing of the firstand the second potential V_(N1), V_(N2) do not occur. In this case thereis a hard commutation of the half-bridge, i.e., a reactive component ofthe output current flows in the wrong direction. In this case thehalf-bridge should be switched-off after a short delay time that is inthe range of some switching cycles. The second error signal S44 ₂represents a second error scenario for which only the second zerocrossing i.e., the zero crossing of the first potential V_(N1) does notoccur. In this case a non-zero-voltage-switching-operation may occur. Inthis case the half-bridge should also be switched off, because in such anon-zero-voltage-switching-operation increased power losses occur. Thetime period between detecting such an error scenario and switching offthe half-bridge may, however, be longer than the time period betweendetecting a hard commutation and switching off, and may amount severalseconds or several 10.000 switching cycles.

Instead of generating the first and second error signals S44 ₁, S44 ₂using the first and second comparator signals S41, S42, there is alsothe possibility of evaluating the voltages V51 ₁, V51 ₂ in theevaluation circuit 43 according to FIG. 13. There is, for example, thefirst error scenario if the slowly increasing voltage V51 ₁ rises abovea first upper threshold value, and there is, for example, the seconderror scenario, if the faster increasing voltage V51 ₂ rises above asecond upper threshold value. These threshold values may be different,but could also be made identical.

First and second drive signals S1, S2 as they are generated by the drivecircuit according to FIG. 10, are not directly suitable for driving thehalf-bridge switches 101, 102, namely not in those cases, in which theinput voltage Vin is larger than the signal change of the control signalS1, S2. In this case a driver circuit 70 is required between the drivecircuit 60 and the half-bridge switches 101, 102, that are, for example,power MOSFET, driver circuit 70 being suitable for transferring thedrive signals S1, S2 generated by drive circuit 60 into suitable signalsthat are suitable for driving the half-bridge switches 101, 102. Anexample of such driver circuit 70 is illustrated in FIG. 15. For abetter understanding the half-bridge, the switches of which aren-channel MOSFET in the example illustrated, is also illustrated in FIG.15. Driver circuit 70 includes two driver stages 71, 72: A first driverstage 71 that generates the drive signal S1 for the high-side switch101, and a second driver stage 72 that generates the drive signal S2 forthe low-side switch 102. The drive signal for the second switch 102generated by the drive circuit 60, and being denoted with S2′ in FIG.15, is directly provided to the second driver stage 72.

For driving the high-side switch a drive signal S1 is required that isrelated to the electrical potential at the output 102 of thehalf-bridge, and that for switching on an n-MOSFET needs to be higherthan the upper supply potential Vin. This drive signal S1 is generatedby the first driver stage 71 which for this is, for example, connectedwith the output 103 and that, for example, includes a bootstrap circuitor a charge pump (not illustrated). For transmitting the drive signalS1′ of the first semiconductor switch 101 generated by the drive circuit60 to the first driver stage 61 a transformer 73 having a primarywinding 73 ₁ and a secondary winding 73 ₂ is present. Transformer 73transmits the drive signal S1′, that is, for example, related to thefurther reference potential or reference potential GND, respectively, tothe first driver stage 11, first driver stage 11 receiving the potentialat the output 103 of the half-bridge as a reference potential, forexample. A drive signal S1′ that is, for example, rectangular is changedin its waveform when transmitting it via transformer 73, so that in thefirst driver circuit 73 demodulation circuit may be present in a mannernot illustrated in detail, these demodulation circuits being suitablefor transferring the signals received at the secondary side of thetransformer 73 in suitable drive signals for driving the firsthalf-bridge switch 101. Such demodulation circuits are commonly known,so that no further explanations are required in this regard.

Transformer 73 is, for example, a air-cored transformer or transformerwithout transformer core (coreless transformer). Such transformer doesnot have a transformer core and can therefore be integrated in or on anintegrated circuit easily.

FIG. 16 by way of a cross section through a semiconductor body 200illustrates a possible realization of an air-cored transformer 73. Theprimary winding 73 ₁ and the secondary winding 73 ₂ of this transformer73 are dielectrically insulated from one another by a transformerdielectric 73 ₃. One of the windings, in the example the secondarywinding 73 ₂, is, for example, arranged at the semiconductor substrate201 below the dielectric 73 ₃ while the second winding is arranged abovethe transformer dielectric 73 ₃. In a manner not illustrated in detailadditional circuit components of the driver circuit 70 or of the drivecircuit 60 may be integrated in the semiconductor substrate 201.

The input voltage Vin of the half-bridge may amount up to several 100Vdependent on the kind of load to be driven by the half-bridge. Drivecircuit 60 and dead time signal generation circuit 10 are, however, forexample logic circuits having a supply voltage or a voltage blockingcapability, respectively, of only some volts. The voltage across thesecond capacitance 22 of the capacitive voltage divider should thereforenot be higher than the voltage blocking capability of these logiccircuits, so that across the first capacitance 21 of this voltagedivider the major part of the input voltage Vin drops. This firstcapacitance 21 of the capacitance voltage divider that is alsoillustrated in FIG. 15 should therefore have a sufficient voltageblocking capability. This is, for example, achieved by using as thecapacitance dielectric the same dielectric as the transformer dielectric73, or that as the dielectric for the first capacitance 21 a dielectricis used that is separated from the transformer dielectric 73, but thathas been produced using the same process steps as the transformerdielectric 73.

For a better understanding the first capacitance 21 of the capacitivevoltage divider is also illustrated in FIG. 16. This first capacitance21 is realized as a plate capacitor in the example illustrated, thecapacitor having two capacitor plates 21 ₁, 21 ₂ that are separated fromone another by dielectric 21 ₃. Dielectric 21 ₃ and the transformerdielectric 73 ₃ may be realized as a continuous dielectric layer, butmay also be separated form one another (illustrated in dash lines). Thedielectrics 73 ₃, 21 ₃ are, for example, comprised of an imide or anoxide.

The overall drive circuit illustrated in FIG. 15, having the drivecircuit 60, the dead time signal generation circuit 10, the capacitivevoltage divider 21, 22 and driver circuit 70, may be integrated in twodifferent semiconductor chips or semiconductor bodies, respectively,that are illustrated in dash-dotted lines in FIG. 15 and that havereference number 200 and 300. In the example illustrated first driverstage 71, transformer 73, and the first capacitor 71 of the capacitivevoltage divider are integrated in the first semiconductor body 200,while the second driver stage 72, the drive circuit 60, the dead timesignal generation circuit 10 as well as the second capacitor 72 areintegrated in the second semiconductor body 300. In a manner notillustrated in detail it is also possible to integrate all componentsexcept for the first driver stage 71 in the second semiconductor body100 and to only integrate the first driver stage 71 in the firstsemiconductor body 200.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. A circuit arrangement for determining a change with time of an outputvoltage of a half-bridge circuit during a dead time, the half-bridgecircuit comprising two switching elements that alternatingly assume anon-state and an off-state and that both assume an off-state during thedead time, the circuit arrangement comprising: a first input forapplying the output voltage; a capacitive network having a first and asecond circuit node each being capacitively coupled to the input, andhaving a terminal for a reference potential; a recharging circuit forthe capacitive network that is configured, during the switched-on phaseof one of the first and second switching elements, to adjust electricalpotentials of the first and second nodes, the electrical potentials eachbeing different from the reference potential; and a comparatorarrangement that is configured during the dead time to determine a timedifference between such times at which the electrical potentials at thefirst and second node each assume a given potential value, this timedifference being a measure for the change with time of the outputvoltage, wherein the capacitive network further comprises: a thirdcapacitance coupled to the tap of the capacitive voltage divider; thefirst circuit node being formed by the tap of the voltage divider, andthe second circuit node being formed by a terminal of the thirdcapacitance facing away from the tap, wherein the changeover circuitfurther comprises: a fourth capacitance, which by the first switch isalternatingly connectable to the second circuit node of the capacitivenetwork, or the terminal for the reference potential; and a secondswitch being connected in parallel with the third capacitance of acapacitive network.
 2. The circuit arrangement of claim 1, wherein therecharging circuit is configured to during the switched-on phase of theone switching element of the half-bridge to close the second switch fora time period that is shorter than the switched-on phase, and tosubsequently open the second switch, during the switched-on phase of theone switching element of the half-bridge, and after opening the secondswitch to connect the third capacitance of the capacitive network to thefourth capacitance of the recharging circuit.
 3. A circuit arrangementfor determining a change with time of an output voltage of a half-bridgecircuit during a dead time, the half-bridge circuit comprising twoswitching elements that alternatingly assume an on-state and anoff-state and that both assume an off-state during the dead time, thecircuit arrangement comprising: a first input for applying the outputvoltage; a capacitive network having a first and a second circuit nodeeach being capacitively coupled to the input, and having a terminal fora reference potential; a recharging circuit for the capacitive networkthat is configured, during the switched-on phase of one of the first andsecond switching elements, to adjust electrical potentials of the firstand second nodes, the electrical potentials each being different fromthe reference potential; and a comparator arrangement that is configuredduring the dead time to determine a time difference between such timesat which the electrical potentials at the first and second node eachassume a given potential value, this time difference being a measure forthe change with time of the output voltage, wherein the capacitivenetwork further comprises: a third capacitance coupled to the tap of thecapacitive voltage divider; the first circuit node being formed by thetap of the voltage divider, and the second circuit node being formed bya terminal of the third capacitance facing away from the tap, whereinthe recharging circuit further comprises: a fourth capacitance, which bythe first switch is alternatingly connectable to the second circuit nodeof the capacitance network or the terminal for the reference potential;and a bridge circuit having four switching elements and having a fifthcapacitance, the bridge circuit being connected between the firstcircuit node and the terminal for the reference potential.
 4. Thecircuit arrangement of claim 3, wherein the recharging circuit isconfigured to during the switched-on phase of the one switching elementof the half-bridge to connect the third capacitance of the capacitivenetwork to the fourth capacitance of the changeover circuit for a timeperiod being shorter than the switched-on phase; during the switched-onphase of the one of the switching elements of the half-bridge to connectthe fifth capacitance of the bridge circuit between the first circuitnode of the capacitive network and the terminal for the referencepotential for a time period being shorter than the switched-on phase,namely with a first polarity, if the one of the switching elements isthe first switching element, and with a second polarity, if the one ofthe switching elements is the second switching element.
 5. A drivecircuit for a half-bridge circuit having a first and a second switchingelement and having an output, the drive circuit comprising: a drivesignal generation circuit that is adapted to generate a first drivesignal for the first switching element and a second drive signal for thesecond switching element dependent on a dead time signal; a dead timesignal generation circuit, comprising: an input for applying the outputvoltage thereto; a capacitive network having a first and a secondcircuit node, each being capacitively coupled to the input, and having aterminal for a reference potential; a recharging circuit for thecapacitive network, the recharging circuit being configured to duringthe switched-on phase of one of the first and second switching elementsto adjust electrical potentials at the first and second circuit nodesthat are each different from the reference potential; a comparatorarrangement that is configured to during the dead time to determine atime difference between those times at which the electrical potentialsat the first and second nodes each assume a given potential value, thistime difference being a measure for a change with time of the outputvoltage, wherein the drive signal generation circuit comprises: a logiccircuit that receives the dead time signal; a first driver circuit,being coupled to the logic circuit via a transformer, and providing thefirst drive signal; a second driver circuit, being connected to thelogic circuit, and providing the second drive signal, wherein: the firstdriver circuit, the transformer and the first capacitance of thecapacitive voltage divider are integrated in a first semiconductor chip;the logic circuit, the second driver circuit and the second capacitanceof the capacitive voltage divider are integrated in a secondsemiconductor chip.
 6. The drive circuit of claim 5, wherein thecapacitive network comprises a terminal for a further referencepotential.
 7. The drive circuit of claim 6, wherein the capacitivenetwork comprises: a capacitive voltage divider having a first and asecond capacitance that are connected in series with each other betweenthe input and the terminal for the further reference potential, andhaving a tap.
 8. The drive circuit of claim 7, wherein the capacitivenetwork further comprises: a third capacitance being coupled to the tapof the capacitive voltage divider.
 9. The drive circuit of claim 5,wherein the transformer is an air-cored transformer having a primarywinding and a secondary winding that are separated from one another by adielectric layer, and wherein the first capacitance comprises a firstand a second electrode that are separated from one another by a seconddielectric layer.
 10. A drive circuit for a half-bridge circuit having afirst and a second switching element and having an output, the drivecircuit comprising: a drive signal generation circuit that is adapted togenerate a first drive signal for the first switching element and asecond drive signal for the second switching element dependent on a deadtime signal; a dead time signal generation circuit, comprising: an inputfor applying the output voltage thereto; a capacitive network having afirst and a second circuit node, each being capacitively coupled to theinput, and having a terminal for a reference potential; a rechargingcircuit for the capacitive network, the recharging circuit beingconfigured to during the switched-on phase of one of the first andsecond switching elements to adjust electrical potentials at the firstand second circuit nodes that are each different from the referencepotential; a comparator arrangement that is configured to during thedead time to determine a time difference between those times at whichthe electrical potentials at the first and second nodes each assume agiven potential value, this time difference being a measure for a changewith time of the output voltage, wherein the drive signal generationcircuit comprises: a logic circuit that receives the dead time signal; afirst driver circuit, being coupled to the logic circuit via atransformer, and providing the first drive signal; and a second drivercircuit, being connected to the logic circuit, and providing the seconddrive signal, wherein the first driver circuit is integrated in a firstsemiconductor chip; the transformer, the first capacitance of thecapacitive voltage divider, the logic circuit, the second driver circuitand the second capacitance of the capacitive voltage divider areintegrated in a second semiconductor chip.
 11. The drive circuit ofclaim 10, wherein the transformer is a air-cored transformer having aprimary winding and a secondary winding being separated from one anotherby a first dielectric layer, and wherein a first capacitance comprises afirst and a second electrode being separated from one another by asecond dielectric layer.
 12. The drive circuit of claim 11, wherein thefirst and the second dielectric layers are formed by a common dielectriclayer.